1. Field of the Disclosure
This disclosure relates to the field of inhibiting the formation of deposits inhibiting the flow of fluid in conduits and the like and, more specifically, to methods and devices for inhibiting the formation of unwanted deposits in downhole production equipment.
2. Description of the Related Art
The problem of unwanted solid deposition in oil wells, gas wells, surface production equipment, and in hydrocarbon flow lines has presented a challenge to the petroleum industry since the first wells were drilled more than one hundred years ago. Although scale deposition is a major problem that interferes with the production of oil and gas, it is not the only problem. Paraffin or wax deposition has also been recognized as a major problem from the inception of the oil industry all over the world, as has asphaltene formation. The occurrence of these unwanted deposits in hydrocarbon producing conduits and related equipment can result in numerous problems, including reduced production and severe and often costly startup problems following pipeline shut down. Other problems with unwanted deposits can include congealing hydrocarbons, interface problems, depositions in tank bottoms, high line pressures, plugged flow lines, under deposit corrosion, plugging of injection wells and filter plugging.
Scale deposit and accumulation is a significant problem to oil and gas producer wells. The rate at which scale accumulates is dependent upon a variety of factors, including the quantity of minerals transported in the fluid, the temperature variations in the well bore, and pressure variations in the tubing, including variations resulting from tubing interior diameter changes. Once scale crystals begin to precipitate out of the fluid and form on the interior of the production conduit, the growth rate can accelerate. This phenomenon has been described as crystalline growth theory.
Chemical treatment methods for the removal of unwanted deposits such as scale, paraffin, asphaltene and hydrates, include acid treatments or the use of a variety of other chemicals to remove the unwanted deposits. Often, the type of chemical treatment method selected will vary depending upon the type of condensate or deposit. Chemicals, such as polyelectrolytes, phosphonates (such as DETPMP), polyphosphinocarboxylic acids (PPCA), organophosphonic acids (such as diethylenetriamine penta(methylphosphonic acid) and hexamethylenediamine tetramethylene phosphonic acid (HMDP)), and polymers such as polyacrylate (PAA), polyvinyl sulphonate (PVS), sulfonated polyacrylates, phosphomethylated polyamines (PMPA), and the ACUMER™ polymer products, such as ACUMER™ 2100, a carboxylate/sulfonate copolymer commercially available from Rohm and Haas Company (Philadelphia, Pa.) are often used to inhibit or prevent the growth of unwanted hydrocarbon deposits, such as scale crystals, on production tubing interiors. Other chemical-related treatments include the use of bacteria, enzymes, and continuous or batch down hole chemical injection and squeeze treatments of crystal modifiers. Typically, such chemicals are effective towards and limited to only specific types of deposits.
Despite their advantages, chemical treatments are usually expensive, environmentally hazardous in many cases, and are oftentimes very sensitive, working effectively only on specific crudes or on specific types of unwanted deposits. Chemical treatment often requires dedicated equipment to introduce the chemicals to the deepest sections of the well bore. Traditionally, scale prevention chemicals are injected down the annulus of the production tubing and enter the production tubing through sliding sleeves or other valves. In recent years, small stainless steel lines have been installed into the interior of the production tubing and run to the deepest point in the well bore. Scale prevention chemicals are pumped through the small line under pressure and mixed with the fluids produced from the well. This allows the fluid to be treated during normal production of the well, but requires continuous monitoring of the injection strings to maintain proper operation. Additionally, operation of the well is further complicated because access to the center of the production tubing is blocked, preventing through tubing, such as wire line or coiled tubing. Treatment chemicals are typically not recoverable from the production fluid.
Some deposits are so hard that chemicals are not effective, requiring physical methods for their removal, including mechanical removal. Physical methods have been studied and put to use for the past several decades as an alternative to chemical methods and to prevent and control unwanted deposit formation. Mechanical removal can include the use of drills, mills and other tools to grind or tear the deposits loose from the interior of the production tubing walls. Occasionally, such processes cause damage to the interior of the tubing and can cause worse scale accumulation rates in the future as a result. In worst-case scenarios, the production tubing must be extracted and replaced. Other physical methods which have been described include hot water circulation, steam injection, cutting or wire-lining, and the use of magnetic devices on electromagnets, such as solenoids and yoke-based electromagnets. However, while electromagnets can produce magnetic fields of great intensity, their choice for use in downhole environments is often not practical since electromagnets require an electrical power supply, cooling, and periodic servicing.
In contrast to electromagnetic devices, permanent magnet devices do not require an electrical power supply downhole and require little to no maintenance. Several attempts have been made to use permanent magnet devices to reduce downhole buildup. Examples of several of the attempts include U.S. Pat. No. 3,228,878 which issued to Moody on Jan. 11, 1966 and discloses the use of magnets to provide a magnetic field having two polar zones a short distance from each other. The field may be provided by one or more high strength permanent magnets located outside the flow passageway and each having its poles facing toward the passageway in a direction normal to its path of flow. The magnetically treated liquid may flow with a minimum of turbulence and free it from external magnetic influence for a distance within the flow passageway from 10 to 150 times the length of the magnetic field to avoid too rapid a dissipation of the change effected therein by the passage through the magnetic field.
Another contribution to the art was made by Debney, et al. in U.S. Pat. No. 4,422,934, which proposes a magnetic device for the treatment of calcareous fluids. Described therein is a device for magnetically treating liquids to inhibit the deposit of scale in plumbing systems, appliances, boilers, and the like. The device has an elongate housing with an inlet and an outlet for the flow of liquid there through. A support structure is located inside the housing to retain a plurality of longitudinally spaced-apart magnets. The magnets are held in position by a plurality of transverse holding elements which are positioned so that the magnets are angularly disposed in a helical arrangement. The magnets are directly immersed in the liquid flowing through the device.
As a further example, U.S. Pat. No. 5,178,757 to Mag-Well, Inc. describes a device that includes an elongated hollow core providing at least one passage through which the fluid to be treated flows. An array of magnets extends longitudinally along the core with the poles of the magnets arranged so as to provide a magnetic field perpendicular to the flow path to enhance the magnetic conditioning effect of the tool. An alternative embodiment of the device has three longitudinally extending arrays of magnets with two fluid passages between them. The magnets are formed of a rare earth magnetic material, and are backed by a flux-carrying member of cobalt-iron alloy, with rounded corners so as to reduce loss of a magnetic field. Each magnet is mounted at least partially within an outer surface of the core with the flux-carrying member contacting, covering, and extending between the outer major faces of the magnets.
U.S. Pat. No. 5,052,491 issued to Harms, et al. on Oct. 1, 1991 describes the use of coupling devices that contain magnets to control the accumulation of paraffin and deposits in a downhole oil string or oil transmission flow lines. The coupling devices are made of a nonmagnetic material surrounded by a magnet and shield of magnetic material. The devices are used to join sections of oil string pipe together which form the downhole oil string casing. The magnetic coupling devices are placed at every 1,000 to 1,500 feet.
U.S. Pat. No. 5,453,188 issued to Florescu, et al. on Sep. 26, 1995 suggests an apparatus and method for preventing and minimizing the formation of deposits of paraffin, asphaltene and scale on the inside of downhole oil string line and on the surface of flow transmission lines. Successive magnet pairs are provided in magnetic discs along a section of pipeline. Each successive pair of magnets is rotated through a particular angle relative to the adjacent pair of magnets to achieve an advantageously prolonged trajectory of charged particles that populate the flowing fluid.
U.S. Pat. No. 5,700,376 issued to Carpenter on Dec. 23, 1997 describes an apparatus and method including first and second housing halves which are welded together to attach the apparatus to a pup joint installed in an oil casing. The housing includes a cylindrical portion and first and second frustoconical portions at opposite axial ends thereof. Axially extending L-shaped spacers are secured to the inside portion and include longitudinal edges which abut with the outer surface of the pipe. Series of axially spaced, first and right parallelepiped shaped magnets are sandwiched between the inside portion of the cylindrical portion and the outer surface of the pipe, with the poles of the first and magnets being reversed relative to the pipe. The housing halves are welded along their longitudinal free edges after being clamped together by a clamping band with sufficient force to secure the apparatus to the pipe generally by frictional forces and being free of the attachment to the pipe, and are secured along the casing pipe at approximately 1,000-foot intervals.
A Federal Technology Alert produced for the U.S. Dept. of Energy by Battelle Columbus Operations in January 1998 discloses the use of magnetic or electromagnetic scale control on a pipe through which water is flowing. It also discloses that manufacturers have applied the technology to petroleum pipelines to prevent wax build-up. A variety of other studies regarding the use and mechanisms of the use of magnets in treating scale, paraffin and asphaltene during petroleum production, including those by Farshad, F. F. et al. [SPE paper No. 77850, 2002; and, SPE paper No. 76767, 2002], and Tung, N. P., et al. [SPE paper No. 68749, 2001].
Although the use of magnetic scale prevention has proven effective for both residential and commercial applications at or near the surface, magnetic scale prevention for down-hole oil and gas production tubulars has been problematic. Lack of success in down-hole magnetic scal prevention has several contributing factors, including a lack of understanding of the fluid dynamic characteristics that exist during normal production of a producing oil and gas well and improper use and configuration of the technology.
For example, magnets have been clamped on the exterior of the production tubing as the production tubing being run into the wellbore. In this configuration, the clamps extend outside of the outer diameter of the tubulars and come in contact with the sides of the well-bore and debris in the annulus between the well-bore and the production tubular. The clamps can become jarred or dislodged during the installation of the production tubing, which allows the magnetic scale assembly to become separated or torn away from the production tubulars. Thus, these clamps can become lost or stuck in the wellbore and then require additional expensive fishing operations for their recovery. The protrusion of magnets on the exterior of the tubing will also limit the ability of the magnets to be conveyed into the wellbore or reservoir in a pressurized condition. This pressurized deployment is referred to as snubbing or stripping into the well. This stripping or snubbing is generally accomplished by the use of elastomers or rubber sealing elements which provide a seal on the exterior of the production tubing as it is pushed or lowered in and out of the well-bore. Snubbing or stripping requires that the outside diameter of the tubing or conduit be smooth to prevent oil, gas or hydrocarbons from being released into the atmosphere during this insertion. The uncontrolled release of oil, gas or hydrocarbons into the atmosphere is referred to as a blowout and, in some scenarios, may result in an explosion or fire. Therefore the use of any assembly that cannot provide a smooth exterior that would allow for these elastomers to seal on would not be recommended by those skilled in the art of oil and gas well servicing. It is always preferred in oil and gas well servicing, whether snubbing or stripping is being performed or not, to maintain the ability to seal on the exterior of the production tubing, since the ability to seal on the exterior of the tubing can be used to trap or contain pressure should the well begin to flow unexpectedly.
In another prior art embodiment, small individual magnets were placed into a subassembly (also referred to as a sub) that is placed between two joints of tubing. Although this configuration eliminates the clamps, the size of the magnets are limited by the interior diameter of the casing and the exterior dimension of the production tubing, and, thus, only smaller, lower strength magnets may be used. In an attempt to compensate for the loss in strength due to the smaller dimension of the magnets, the subs were made out of a nonferrous material. Although the use of nonferrous subs can reduce distortion and magnetic field strength losses, the strength of the magnets proved to be ineffective. This is further complicated when many small magnets having the same polarization are placed side by side. The alignment and the natural repelling effects generated by magnets with the same polarization in proximity to one another causes great distortion in the field of magnetic flux generated by the individual magnets. Additional energy is lost from the already limited strength of the magnets, and the field of magnetic flux becomes heavily distorted. Thus, uniform penetration of the tubular with the magnetic field and the energy transfer to the fluid is not fully accomplished. Additionally, this prior art embodiment did not give consideration or provide mechanisms to change the interior velocity of the fluid as it passes through the magnetic field.
Recent research has shown that, for magnets to be effectively used in the prevention of scale, the interior fluid velocity must be maintained at a minimum level, or critical velocity. When fluid velocities drop below this critical velocity, the proper ion arrangement does not occur. Previous prior art embodiments provide no mechanism for fluid acceleration through the magnetic fielding beyond the natural velocity maintained by the interior diameter of the production tubing. This is due to a lack of understanding of the velocity and or production mechanics of oil and gas wells production rates. For those skilled in the art of production recovery, it is understood that velocities or production rates will vary from well to well and will change throughout the life of a single well. This generally occurs when the well begins to lose pressurization or become depleted over time as the oil is produced. This loss of pressurization will further result in a decrease in the well's production velocity.
Research has shown that proper polar alignment of the magnets must be maintained to keep the particles in the tubular contained within the fluid. Incorrect polar alignment results in the acceleration of scale deposition. It has been firmly established in the scientific world, that the positive, magnetic flux field influence of the South Pole changes the adhesion characteristic of liquids making them become more soluble. This occurs when the ions are arranged as they pass through the magnetic field of north to south orientation. The positive effect of the South Pole will repel the positively charged particles contained in the fluid. This repelling effect will cause the particles to change from a random arrangement to a structured arrangement. This effect is referred to as Kronberg Platet Formation. By arranging the magnetic field so as to pass through the positive or South Pole last, the positive sides of the particles are furthest from the negatively charged piping. This realignment of the ions then carries the positive charge from the south polarization. This retained magnetic charge is referred to as magnetic memory effect. This memory or charge has been measured in static bodies of fluid up to one year from the induction. However, consideration must be given to the discharge or loss of magnetic polarization that occurs as fluid is transported through long intervals of piping. This discharge occurs due to turbulence in the fluid, wherein the ions are shuffled and the net charge is lowered. The disruption in the magnetic memory is referred to as Vibrational Depolarization. Vibrational depolarization occurs when a fluid that has had a charge induced into is affected by the turbulent effect of the pipe or conduit it is being moved through. The greater the turbulence of the fluid the quicker the polarization or charge is lost. Due to vibrational depolarization, the magnetic memory of the particles must be reestablished at intervals no greater than 250 feet, in order to keep particles contained within the fluid medium and prevent scale deposition. At these intervals the charge has proven effective to keep the particles in the fluid from precipitating out and forming scale. It has also been shown, where scale deposits already exist, reestablishing the field at intervals of about 165 feet can attract particles back into the fluid medium, removing at least part of the scale deposits from the tubular walls and thereby causing a reduction of the existing scale. This occurs when the particles in the fluid have a stronger induced polarity than the particles have to other scale crystals or the tubing walls itself.
Additionally, most prior art embodiments fail to take into account the extreme bottom hole temperatures that may exist in oil and gas wells. It is generally known by those skilled in the art of oil and gas production that scale precipitation can be most severe on the wells that have the highest bottom hole temperatures and pressures. It has been shown that magnets degrade or lose strength more rapidly under higher temperature operations. Therefore the use of magnets that have not been properly designed to endure the higher temperatures will result in degradation and failure.
Extreme conditions may weaken or nullify the strength of the magnets before they can influence the particles in the fluids. Magnetic fields are essential in producing a magnetic memory effect in the particles. This positions the particles in the stronger magnetic field generated by the cylindrical magnet for increased lengths of time to insure proper energy transfer to the ion arrangement of the particles. This magnetic memory effect causes the particles (that in effect have become small magnets) to group together, which helps to neutralize their polarity or charge. When the polarity of the particles is neutralized or reduced, the particles tend to remain in the fluid for longer periods of time.
The magnetic memory in the particles may be induced by a magnet is orientated in the wellbore, such as a one-piece cylindrical magnet, so that the fluid passes from a North Pole to the South Pole orientation. This allows the positive charge from the south polar field to be the last to influence the fluid and the particles are before leaving the flux field. It has been shown in the scientific community that South polar effect (positive charge) causes the particles to be less affected by the polarity of the production tubing, therefore maintaining the magnetic memory over greater distances. However this magnetic memory effect can be disrupted as the fluid passes through the interior of the piping over long intervals.
A shortcoming of prior art magnetic deposition prevention systems is that the magnet can be damaged during insertion and/or removal of the production tubular into/from the well due to contact with the inner wall of the casing.
Another shortcoming of the prior magnet deposition prevention systems is that the magnet may be crushed by compression forces along the length of the production tubular or the magnetic retainer.
For these reasons the need to develop magnetic subs designed specifically for scale inhibition of down-hole oil and gas production tubulars exists. There is a need for a downhole magnetic deposition apparatus that protects the magnet from longitudinal compression forces and damage from contact with the casing.